fix: add tab handling rules to prevent infinite loops

[Jah-yee]

Fixes issue #201: Agent gets stuck in infinite loop when clicking links that open in new tabs.

Problem

The current system prompt explicitly forbids clicking links that open new tabs but provides no guidance on how to handle them properly.

Solution

Added proper tab handling instructions to both system prompt files:

1. packages/core/src/prompts/system_prompt.md - Updated capability section
2. packages/extension/src/agent/system_prompt.md - Added tab_handling_rules section

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